In order to reduce the rate of corrosion of metals, and particularly metals containing iron, from one or more metal corrosion agents present in a fluid (i.e., a gas, liquid, slurry or a mixture thereof) a corrosion inhibitor is frequently introduced into the fluid to reduce the rate of corrosion of the metal vessel, pipeline and/or equipment used to store and transport the fluid. In oil and gas-field applications, for example, corrosion inhibitors are added to a wide array of systems, including without limitation, cooling systems, refinery units, pipelines, steam generators and oil or gas producing units in efforts to combat a variety of types of corrosion.
One example of corrosion, among others, typically encountered in the transport of a fluid containing one or more corrosion agents (hereinafter simply referred to as "fluid") is flow-induced corrosion. In the case of flow-induced corrosion, the degree of corrosion that occurs is presently believed to depend on a variety of factors, including the corrosiveness of the fluid itself, the metallurgy of the pipeline and the shear rate, temperature, and pressure of the fluid. Also, to the extent that a corrosion inhibitor is used, the inhibitor's ability to reduce the rate of corrosion of a metal from flow-induced corrosion, among other types of corrosion, is presently believed to depend on at least two factors. One factor is the inhibitor's chemical affinity for the metal surface. A second factor is the inhibitor's resistance to breakdown under high shear conditions. Therefore, it is currently believed that the rate of corrosion, especially flow-induced corrosion, of a metal more likely will be reduced where the inhibitor has good chemical affinity for the metal surface and can resist breakdown under high shear conditions. Many inhibitors have been developed to reduce corrosion. However, their activity is sufficiently low that higher concentrations are oftentimes required to effectively treat a pipeline, most particularly where flow-induced corrosion is a problem, thereby increasing operating costs.
Also, where a particular inhibitor shows good corrosion inhibition activity, it typically costs significantly more to manufacture than alternative inhibitors with lower activity. For example, quaternized imidazolines like those disclosed in U.S. Pat. No. 5,322,640 can be produced with, among other compounds, tall oil fatty acid (TOFA) and an alkyl polyamine, also often referred to as a polyalkylene polyamine, such as diethylenetriamine (DETA). It is well understood by those skilled in the art that the production of the amino imidazoline desired for synthesizing preferred imidazoline derivatives useful for corrosion inhibition is produced by reacting stoichiometric amounts (i.e., a 1:1 mole ratio) of a high molecular weight monocarboxylic fatty acid having from 16 to 32 carbon atoms with an alkyl polyamine (see e.g., U.S. Pat. Nos. 3,687,847 and 3,758,493). Often an excess amount of alkyl polyamine (for example, a TOFA:DETA mole ratio of about 0.77:1) has been used in previous commercial applications to produce the amino imidazoline.
The presence of a free amine moiety enhances the reactivity of the pendant alkyl amine group versus the unsubstituted nitrogen atom in the imidazoline ring. Various imidazoline derivatives are produced typically by reacting the imidazoline with organic carboxylic acids, such as, for example, acrylic acid (CH.sub.2 CH.sub.2 COOH), which preferably react with the imidazoline's pendant alkyl amine group, to enhance the its corrosion inhibition activity by increasing its partitioning into water.
Conventionally, the 1:1 TOFA: DETA mole ratio has been considered desirable because it was heretofore thought to yield a substituted imidazoline with a pendant alkyl amine group that has at least one free amine (e.g., a NH.sub.2 group) available for interaction with a metal surface. However, the Applicant has discovered that, unexpectedly and surprisingly, the reaction product produced with such low TOFA:DETA ratios (i.e., from about 0.6:1 to about 1.2:1) can produce a reaction product mixture comprising both an amino imidazoline and an amido imidazoline, although the amido imidazoline is believed to be produced at a lower mole% than the amino imidazoline. Heretofore, it was believed by those skilled in the art that the only imidazoline formed would be an amino imidazoline in the reaction product where a low TOFA:DETA mole ratio was used in the synthesis process. Moreover, it was believed that using higher TOFA:DETA mole ratios, in the range of from about 1.3:1 to about 500:1, would lead to the production of excess amounts of amido type imidazolines. Conventionally, such amido imidazolines and their derivatives generally were considered to have little to no corrosion inhibition activity or potentially to have detrimental effects on the reaction product's corrosion inhibition activity. Put another way, amido type imidazolines were considered an impurity or contaminant in the reaction product because they lacked a pendant group with a heteroatom (e.g., nitrogen, sulfur or oxygen) having a pair of nonbonding electrons freely available for interaction with a metal surface.
Accordingly, until the disclosure of the present invention, those skilled in the art of synthesizing corrosion inhibitors refrained from reacting higher mole ratios of a monocarboxylic acid (e.g., TOFA) with an alkyl polyamine (e.g., DETA) and/or producing imidazoline derivatives where the group pendant to the imidazoline ring contains an amido moiety, generally described as --N--(C.dbd.O)--R. In this instance, the pair of nonbonding electrons on the nitrogen atom of the pendant group would have a preferential affinity for the proximate carbonyl moiety over that for the metal surface. In turn, it was thought that this absence of a freely available pair of nonbonding electrons would reduce the compound's ability to interact with a metal surface, and thereby reduce its overall inhibition activity.
The cost of alkyl polyamines, such as DETA, is high (e.g., $1.50/lb.) as compared with a monocarboxylic fatty acid, such as TOFA (e.g., 24.cent./lb.). Consequently, the use of certain imidazolines for reducing the corrosion rate of a metal, most particularly for improving its resistance to flow-induced corrosion, can lead to increased operating costs. A comparatively lower cost inhibitor is desired that has corrosion inhibition performance comparable to or better than inhibitors presently used for treating systems experiencing flow-induced corrosion, among other metal corrosion problems.
A substantial number of corrosion inhibitors have been disclosed for reducing the rate of corrosion of metal-containing storage and transport systems. More specifically, a number of corrosion inhibitors have been disclosed most particularly for treating flow-induced corrosion, including, among others, quaternized imidazolines. However, these imidazolines are relatively costly to manufacture. Accordingly, a need exists for a corrosion inhibitor that is less costly to manufacture compared to such known inhibitors.